21 8 CHORDATA 



higher fishes developed from the primitive type through the in- 

 fluence of a rapid stream environment is the belief of T. C. Chamber- 

 lain. 



Arthropod Theories. — The theory of Gaskell (1908) is based on 

 the assumption that the whole alimentary canal of a crustacean-like 

 Arthropod united with the nerve cord to form the hollow brain and 

 spinal cord of the vertebrates. Gaskell's theory reflects his knowl- 

 edge of physiology, but indicates his lack of training in comparative 

 anatomy, embryology and paleontology. 



Patten's Arachnid theory was first published in 1889 in the 

 Quarterly Journal of Microscopical Science, and was followed by a 

 number of important papers preceding the publication of his book 

 in 191 2. Dr. Patten has pointed out (personal communication, 

 1 931) that the Arachnid theory gives a satisfactory explanation 

 of the other theories of evolution of vertebrates while none of these 

 theories explains the detailed resemblance between Vertebrates and 

 Arachnids, 



The following summary was furnished by Dr. Patten (consult 

 his article in the Quarterly Journal of Microscopical Science, vol, 

 Si.PP- 317-378, 1890): 



"The Arachnid theory — 1890 — is based on the following evi- 

 dence: (i) In the Arachnids, the first sixteen, or more, metameres 

 form a characteristic pattern, consisting of five groups of highly 

 specialized functions and organs, all of them different and all ar- 

 ranged in a definite sequence. (2) This basic pattern is essentially 

 the same as that in the "head" of vertebrates, and all the corre- 

 sponding organs in each group have essentially the same structure, 

 and develop embryologically in essentially the same ways. Some 

 of the corresponding parts are the notochord and the endocranium, 

 the main divisions of the brain, special groups of sense organs, 

 nerves, ganglia, somites, gill sacs, and oral arches. (3) The marine 

 Arachnids were the highest animals in existence during the long 

 and very early geologic eras. In the Cambrian, or some pre-Cam- 

 brian era, they apparently gave rise to the great class of Ostraco- 

 derms, which are mainly Silurian; and they in turn gave rise to the 

 true fishes, with united oral arches, which first appeared in the 

 Devonian. Many peculiarities of the Ostracoderms, especially 

 the structure of the exoskeleton and the oral arches, support this 

 conclusion. (4) The Tunicates, Amphioxus, and other chordate 



